This invention relates to the art of nuclear reactors which serve as prime energy sources for power plants. It has particular relationship to the processing of the nuclear fuel assemblies of such reactors to remove, replace or rearrange their fuel rods. While this invention is conceived for the purpose of processing fuel assemblies and is uniquely suitable for this purpose, it may be applied for other purposes. The application of this invention to such other purposes is regarded as within the scope of equivalents of this invention as scope of equivalents is defined and explained by the United States Supreme Court in Grover Tank & Mfg. Co. v. Linde Air Products 339 US 605; 94 L.Ed. 1097.
The individual fuel rods of a nuclear reactor are mounted in fuel assemblies. Each fuel assembly includes a skeletal frame or skeleton in which the rods are held by grids. A plurality of tubes for receiving control rods are interspersed among the rods. These tubes are usually called thimbles. Typically, the fuel rods are arrayed in a fuel assembly in a square, 17 fuel rods on each side. There are typically 264 fuel rods and 24 thimbles in each square. Typically each fuel assembly is 14 feet in length. Each fuel assembly includes a bottom nozzle and a top nozzle. The nozzles have holes and the coolant enters the assembly through the bottom nozzle and exits the assembly through the top nozzle. The bottom nozzle are secured to the skeleton and specifically to the thimbles generally by screw fasteners which are screwed into the thimbles and welded or mechanically joined to the bottom nozzle plate. In one type of widely used fuel assemblies the bottom nozzle is secured by screw fasteners each screwed into a thimble and joined to the nozzle plate by a lock-pin pressed into a slot in the head of the fastener and welded to the nozzle plate. Such assemblies are included in many of the reactors which are currently in operation in power plants. Assemblies of alternative structure is disclosed in Application Ser. No. 186,937 filed Sept. 12, 1980 to John M. Shallenberger and Stephen J. Ferlan for RECONSTITUTABLE FUEL ASSEMBLY FOR A NUCLEAR REACTOR and assigned to Westinghouse Electric Corporation now abandoned. This Shallenberger et al application is incorporated hereby by reference. Such assemblies are called reconstitutable fuel assemblies. In reconstitutable assemblies a screw fastener is screwed into the thimble and engages the bottom nozzle plate. A thin cylindrical wall or skirt extends around the head of the fastener and this wall is crimped or swaged into one or more grooves in the nozzle plate. At the top of the fuel assembly each thimble is encircled by a sleeve. The sleeve and thimble are welded to the top of the nozzle plate and are secured together by a bulge below the top.
Fuel rods must be removed from and replaced in fuel assemblies from time to time. Among the typical reasons for the removal and subsequent replacement of fuel rods is the replacement of defective rods, for example, a fuel rod whose cladding is cracked or perforated. It is also necessary to determine if one or more rods of an assembly leaks. In the case of a spent assembly, failed rods should be removed to preclude fission product escape and the attendant hazard. At times a skeleton of a fuel assembly or the assembly itself is damaged. Typically the grids are damaged during reloading or refueling or the grids are damaged by neutron fluence. In the event of such damage it is necessary to remove partially spent fuel rods for transfer to a sound skeleton. It is also necessary to remove or rearrange fuel rods in a fuel assembly for more effective fuel utilization, i.e., to provide a loose-lattice, a spectral shift or to convert an assembly into a so-called driver fuel assembly. It may also be desirable to replace uranium rods by thorium or plutonium rods.
It is an object of this invention to carry out the removal and replacements of fuel rods from a nuclear fuel assembly effectively and efficiently and to provide apparatus and a method for accomplishing this purpose. It is also an object of this invention to provide a method for replacing a welded nozzle on a fuel assembly by a crimped nozzle.